This invention relates to methods of producing amorphous materials, more particularly to methods for producing amorphous metal phosphates having desirable particulate characteristics for applications involving interactions with metals and ceramics.
Finely divided glass powders, particularly metal phosphates, are useful in the manufacture of coating compositions, conductive metal pastes, cements, as adhesives in bonding metals to ceramics, and for fabricating multilayer ceramic substrates for semiconductor packages. A variety of techniques have been attempted for the production of inorganic powders that are characterized by particles which have fine grain and microsized dimensions. Grinding of organic powders can result in very fine particles but conchoidal fracture and other cleavage effects during grinding will yield particles with sharp edges and jagged contours.
Shock cooling, spray drying, and prilling have also been investigated for this purpose. In most cases these methods provide powder particles that have one or more undesirable physical properties such as porosity, nonuniformity of particle size and shape, lack of crystal homogeneity, and the like.
Metal phosphates are of particular interest because they can be used to bond conductive metals, such as copper, to various ceramic and glassy materials. The reason for its bonding action is that the metal phosphates are compatible with both the metal and ceramic and/or glass substrate. In general, the metal portion of the metal phosphate reacts and bonds to metal pads or metal lines on the substrate, and the phosphate portion is compatible with and reacts with ceramic and glass. Aluminum phosphate, AlPO.sub.4, is particularly adapted to bond copper and glass ceramic.
The metal oxides, and more particularly, the metal phosphates, are effective for performing the bonding function when the surface area of the metal is large, and the particles are non-crystalline, i.e., amorphous in nature. In order to achieve high surface areas, the particles should be very small, on the order of less than 200 Angstroms. The particles, when amorphous, are at a higher energy state than when the particles are crystalline in nature, and are therefore more reactive.
Glassy aluminum phosphate cannot be prepared by melting a mixture of Alhd 2O.sub.3 and P.sub.2 O.sub.5 since the P.sub.2 O.sub.5 volatilizes from melts of aluminum phosphate. Precipitation of aluminum by phosphate in aqueous solution, yields crystalline aluminum phosphate. Glassy aluminum phosphate has been produced by Cassidy et al, Imperial Chemical Industries, in which synthesis aluminum chloride and concentrated H.sub.3 PO.sub.4 are reacted in ethanol at dry ice temperatures. These efforts are set forth in SCIENTIFIC AMERICAN 248 (5), p.114 (1983). This yields a yellow powder, where the yellow color is due to iron contamination, in aluminum chloride. Chloride contaminants are also present. The powder melts to form yellow glass. The necessity of doing the synthesis far below the ambient temperature is also a gross inconvenience. The necessary addition of aluminum chloride, which reacts with the H.sub.3 PO.sub.4, produces yet another corrosive gas, i.e., HCl, which outgasses on pyrolysis.
Sol-gel syntheses are typically performed in one phase. A metalorganic compound is dissolved in the solvent in which the gelling agents, usually aqueous bases or acids, are also soluble. This can yield fine particles if the gellation reaction is slow. If it is a rapid reaction, then uncontrollable particle growth may result.